1. Field of the Invention
This invention relates generally to a position fixing and position reset system for underwater vehicles. More particularly, the invention relates to a position determining system that uses a triaxial vector magnetic sensor to acquire data from a magnet positioned in a search area, and then uses mathematical algorithms to derive from the data a position fix for the vehicle which can be used to reset the vehicle's dead reckoning navigation system.
2. Description of the Prior Art
Some underwater unmanned vehicle missions require the search of an area of water to precisely locate one or more objects. Typically, such vehicles use dead reckoning navigation systems such as an inertial navigation system or a Doppler sonar navigation system. The accuracy of dead reckoning navigation systems deteriorates with time because of unavoidable errors in the outputs of its various components. Eventually, a position reset is required to restore the desired position accuracy. Several methods exist in the art to perform this position reset.
Radionavigation systems could be used to reset the vessel's navigational system. However, since radio waves are rapidly attenuated in sea water, the vehicle must either go to the surface or very near to the surface in order to receive the radio information. Thus, if the vehicle was operating relatively deep, much time and energy would be required for the vessel to travel to the water surface.
Acoustic bottom transponders are another traditional method of providing position fixes to underwater vehicles. However, if transported to the search area by the vehicle and then deployed, transponders significantly reduce the vehicle's available payload. Transponders must also be surveyed once they are deployed to determine their precise bottom location with respect to one another. Typically, the survey process involves repeated and time consuming passes of the vehicle over the area in which the transponders lie in order to gather the necessary acoustic range information. Finally, signals from three transponders are required to determine a unique position fix for the vehicle.